1. Field of the Invention
The present invention relates to wireless telecommunications and, more particularly, to systems and methods that use gateways for tandem free operation.
2. Description of Related Art
Mobile stations often transmit and receive voice and other media in a compressed digital format in order to reduce bandwidth. In particular, a mobile station may receive voice or other media from the user, digitize it, compress the digital signals in accordance with various algorithms, and then transmit the voice or other media in the compressed digital format. A mobile station that receives media in the compressed digital format decodes it in order to obtain the original signal. Mobile stations typically include vocoders to convert the voice or other media between analog and compressed digital formats. A number of different compressed digital formats are commonly used. One example is the Enhanced Variable Rate Codec (EVRC) format, which supports voice transmission at a data rate of about 8 kilobits per second (Kbps). The EVRC format is described in the standard “Enhanced Variable Rated Codec, Speech Service Option 3 for Wideband Spread Spectrum Digital Systems,” TIA/EIA/IS-127 (published January 1997), which is incorporated herein by reference.
In contrast, circuit-switched telephone networks, such as the public switched telephone network (PSTN), typically carry voice or media in an uncompressed digital format, such as a pulse code modulation (PCM) format. For example, in a typical PCM format, a voice signal is sampled at a rate of 8000 times a second and digitized with an 8-bit resolution, resulting in a 64 Kbps signal. Trunks used in the PSTN are often able to carry many of these 64 Kbps signals at a time, in separate channels. For example, a T-1 trunk is able to carry 24 of these 64 Kbps channels.
Because of the differences between these digital formats, wireless telecommunications systems often use transcoders to convert between the compressed digital formats used by mobile stations and the PCM format used by circuit-switched telephone networks. Using transcoders can result in inefficiencies, however. For example, if a call involves two mobile stations, then the voice or other media exchanged in the call may be transcoded twice. One transcoder may convert the media from the transmitting mobile station to PCM format for transmission through the PSTN, and another transcoder may convert the media from the PCM format to the compressed media format for transmission to the receiving mobile station. Using transcoders “in tandem” in this way is often undesirable because each transcoder can add delay to the transmission and can degrade the quality of the voice or other media.
To address these concerns, various proposals have been made to provide “tandem free operation” (TFO) in calls between mobile stations, in which the transcoders are bypassed and the PSTN carries media in the compressed digital format without transcoding. One such approach is described in 3rd Generation Partnership Project 2, “CDMA Tandem Free Operation,” 3GPP2 A. S0004-B v2.0 (Aug. 5, 2002). In this approach, an inband signaling protocol is used to test if the call is a mobile-to-mobile call, that the mobile stations use the same compressed digital format, and that the systems at both end of the call support tandem free operation. If these tests are successful, the inband signaling protocol establishes a “transparent” digital channel through the PSTN, and the transcoders are bypassed. The media is carried through the PSTN in the original compressed digital format, in 16 Kbps “TFO frames” that are mapped onto the two least significant bits in a 64 Kbps PCM frame.
This TFO approach still has a number of disadvantages. First, some network elements in the PSTN may need to be modified to be compliant with this TFO approach. Second, a significant amount of inband signaling may be required to establish the “transparent” connection. Accordingly, there is still a need to provide efficient systems and methods for tandem free operation.